Efficient light coupling between an optical fiber and a silicon waveguide is highly desired for silicon based photonic device and circuit applications. Due to the high refractive index contrast of silicon waveguide systems, obtaining good fiber-silicon waveguide coupling may be challenging.
In optical communication, information is transmitted by way of an optical carrier whose frequency typically is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal. A typical optical communication network includes several optical fibers, each of which may include several channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength.
Technological advances today include optical communication at the integrated circuit (or chip) level. This is because integrated circuits have size advantages that are attractive in computer systems. Sometimes designers couple an optical signal (light) between two chips, between a chip and a die in the system, or between two dies. This is traditionally accomplished using an optical fiber to couple light between waveguides on dies or chips.
One limitation of using the optical fiber to couple light between waveguides on dies or chips is that this method of coupling tends to be inefficient. One reason is because of the physical size difference between the optical fiber and a typical waveguide on a chip or die. The optical fiber tends to much larger than the waveguide. Because of the size difference the optical signal coupling efficiency is poor. That is, the light from the larger diameter optical fiber does not fit well into the small waveguide. The result can be that received light levels are so low that individual bits in the data stream in the optical signal become indistinguishable. When this happens, the receiving component may not be able to recover the information from the data stream.
Coupling efficiency may be improved by attaching lenses to the optical fiber or by placing a lens between the optical fiber and the waveguide to focus the optical signal into the waveguide. However, coupling efficiency is only fair using lenses. Other coupling methods result in efficiencies that are also fair at best.
This limitation also comes with another challenge such as efficient coupling from the optical mode supported by the larger optical fiber to the smaller optical mode supported by the waveguide. The mode is the optical cross-sectional distribution of energy (Gaussian distribution) and is defined by the size of your waveguide (optical fiber, planar waveguide) and the wavelength of the light. There is a large optical mode in the larger optical fiber and a smaller optical mode in the smaller waveguide.
Also coupling from an optical fiber to small on-die waveguides requires very precise alignment. This is typically accomplished with specialized precise manual alignment procedures. Such specialized alignment procedures typically are very expensive and limit practical volumes.
Today, there is a fundamental problem existing for a low-cost multi-mode fiber (MMF) based optical receiver for high speed applications. To achieve high speed, for example 25 Gb/s and beyond, operation for a photo-detector (PD), the active area of the detector is usually required to be small. However, to efficiently couple light from MMF into a semiconductor waveguide based chip that contains photo-detectors and potentially an optical de-multiplexer, a large waveguide size is used for big misalignment tolerance needed for low cost passive alignment.